1. Field of the Invention
The present invention relates to a high-efficiency optical modulator and to a signal converter using the optical modulator that can extract high-frequency radio signals from optical signals intensity-modulated using high-frequency radio signals in the milliwave or microwave range and radiate the extracted signals from an antenna as electric waves, and can also intensity-modulated light using high-frequency radio signals in the milliwave or microwave range.
2. Description of the Prior Art
A passive optical network (PON) uses transmissions over optical fibers to bring signals close to the end user. At the end terminal, the system transmits the signals as radio signals. PON technology is attracting attention as a way of handling broadband Internet transmissions. There is a need for optical modulation technology that, while using compact equipment, is able to use received high-frequency radio signals to modulate optical signals with high efficiency and can efficiently transmit high-frequency radio signals superimposed on the optical signals. As described below, there have already been a number of related basic inventions and announcements.
An effective method of using a received high-frequency radio signal to modulate an optical signal with improved efficiency is to increase the voltage of the high-frequency radio signal applied to the modulator or to increase the length of the optical modulation path of the modulator. Ways have also been devised for reducing the length of the optical path of the modulator. Reference 1, for example, describes an optical modulator configuration comprising a Mach-Zehnder optical modulator in which the optical path is folded at a central perpendicular segment, wherein the incident beam is split into two beams which are phase-modulated, reflected, again phase-modulated and recombined at the original branch point (Ishikawa, xe2x80x9cOptical modulator for two-way transmissions using an LN optical waveguide,xe2x80x9d C-215, p. 4-257, Spring Conference of The Institute of Electronics, Information and Communication Engineers, 1992). In addition to the configuration of Reference 1, Reference 2 describes a modulator in which input high-frequency radio signals are reflected at one end of a divided optical path (W. Burns, et al., xe2x80x9cBroad-Band Reflection Traveling-Wave LiNbO3 Modulator,xe2x80x9d IEEE Photonics Technology Letters, Vol. 10, No. 6, 805-806, 1998). There are also disclosures of an apparatus and communication system able to extract a high-frequency radio signal from an optical signal and radiate the extracted signal from an antenna as a high-frequency radio wave.
Japanese Patent Laid-Open Publication No. Hei 8-508370 discloses a hybrid wireless communication system that uses light and high-frequency radio signals. The system uses a transducer equipped with an electrooptic detector that does not use electric power, and a radio antenna. The electrooptic detector is electrically connected directly to the radio antenna and can receive optical signals via the transducer without using electric power, and transmit radio signals. The transducer can therefore operate without a supply of electricity, such as in a remote region.
Japanese Patent Laid-Open Publication No. 2001-24596 discloses a radio apparatus and communication system in which the radio transmission signals are amplified. High-frequency modulated optical signals sent from a transmitter are converted to high-frequency electric signals by an electrooptic conversion element to which a bias voltage from a bias voltage power supply is applied. The system outputs high-frequency electric signals that is directly applied to a transmit antenna without an amplifier for emitting radio wave. The publication describes an antenna for receiving external electric waves, an optical modulator that, in accordance with the waves thus received, modulates light from the light source transmitted via an outbound path, and an inbound path that transmits the light thus modulated. The publication also describes using a solar battery to generate the bias voltage.
The following disclosures relate to apparatuses that can modulate light using a high-frequency radio signal such as a milliwave or microwave frequency signal received via an antenna.
First, a field sensor able to measure the waveform of an electromagnetic wave that includes a broadband frequency component is disclosed by Japanese Patent Laid-Open Publication No. Hei 5-2043. A source light signal is guided to an optical modulator by an polarization-maintining optical fiber and is modulated by an electro-optic crystal, using an electric field detected by a sensor rod. The modulated signal is guided by a single-mode fiber to an optical detector and converted to an electric signal that is measured by a voltage measuring device.
Japanese Patent Laid-Open Publication No. Hei 9-51307 discloses a modulation apparatus used for information communication applications that performs electro-optical conversion. The apparatus has a small, flat electro-optical conversion section for directly converting radio signals to optical signals. The apparatus includes a substrate that exhibits an electro-optic effect, at least one pair of optical waveguides formed on the substrate that are parallel but ultimately merge, electrodes formed on the optical waveguide that function as an external spatial field signal detector antenna, a light source and optical fiber for supplying light to the optical waveguide, an optical detector for detecting light exiting the optical waveguide, and an optical signal receiver. The apparatus uses an external spatial field signal to directly modulate light that passes through the waveguide.
However, the above disclosures differ from the present invention, as described below.
First, the present invention differs from the References 1 and 2 in that the References do not improve the modulation efficiency by again modulating modulated light reflected back by the optical path. The prior art system disclosed by Published Japanese translation of PCT international publication for patent application(Japanese Patent Laid-Open Publication) No. Hei 8-508370 is configured to convert optical signals to radio signals, but differs from the present invention in that the prior art system is not configured to convert radio signals to optical signals and transmit the converted signals.
The radio apparatus and communication system disclosed by Japanese Patent Laid-Open Publication No. 2001-24596 use a configuration for converting radio signals to optical signals, and thus differs from the present invention with respect to the incorporation of a configuration that converts optical energy to electrical energy.
Similarly, the electro-optical conversion and modulation apparatus of Japanese Patent Laid-Open Publication No. Hei 5-2043 differs from the present invention, which uses a different modulator in converting radio signals to optical signals.
The system disclosed by Japanese Patent Laid-Open Publication No. Hei 9-51307 includes the ability to convert optical signals to radio signals, but differs from the present invention in that it does not include the function of converting radio signals to optical signals.
References 1 and 2 describe an optical modulator configuration in which light that has been modulated is reflected back to the same modulator to be modulated a second time. Also, a resonant type optical modulator is described by Reference 3 (Kawanishi, et al., xe2x80x9cResonant type optical modulator having a planar structure,xe2x80x9d The Institute of Electronics, Information and Communication Engineers [Communication Techniques], 2000-05). However, these do not mention applying the modulators to a transmitters and receivers.
Thus, to date there have been various disclosures such as described above. However, electro-optic signal conversion systems using conventional PON technology employ standing wave type optical modulators, in which the length of the optical path has to be increased in order to obtain adequately modulated optical signals.
An object of the present invention is therefore to provide a high-efficiency optical modulator by using an optical modulator having a high modulation efficiency per unit length of optical path. Another object is to provide an apparatus using the optical modulator that converts between optical signals and electric wave signals.
The present invention uses a configuration in which an optical waveguide is divided into two optical waveguides, each of which is terminated by an optical filter. Light transmitted by one filter is guided to an optical detector having a frequency response that extends up to a high frequency, and light transmitted by the other optical filter is guided to an optical detector having a low frequency response. The output from the former filter goes to a transmission antenna, and the output from the latter filter is used as a DC power supply for the modulator""s bias controller and amplifier. An optical phase modulator is provided midway along the two optical waveguides, forming a folded Mach-Zehnder type of optical modulator configuration. The optical phase modulator is a resonant type. The optical wavelength used when converting optical signals to radio signals is different from the wavelength used when converting radio signals to optical signals. The former wavelength is transmitted by the optical filter and the latter wavelength is reflected by the optical filter. The reflected light again passes through the optical modulator at a time interval that is an inverse integer multiple of the frequency of the high-frequency radio signal, making it possible to maintain a high modulation efficiency. The high-frequency radio signals received via the antenna are amplified and supplied to the optical modulator. The same antenna can be used for reception and transmission.
In accordance with a first aspect of the invention, the above object is attained by an optical modulation apparatus comprising an optical signal input section, an optical path that propagates the optical signals, a branching portion that divides the optical path into a plurality of optical paths, an optical modulator that phase modulates optical signals in at least one of said plurality of optical paths, a wavelength selective filter that selectively reflects and transmits light, a structure whereby an appropriate time within a range having a lower limit obtained by subtracting one-fourth of an integer multiple of a high-frequency radio signal period applied to the optical modulator and an upper limit obtained by adding one-fourth of the integer multiple of said period is equal to twice the time it takes an optical signal to reach the filter from a center of the optical modulator; and a structure whereby an optical signal input from the input section that is divided into a plurality of optical paths at the branching portion, phase modulated by a phase modulator, reflected by the filter back along the optical path and again phase modulated by the phase modulator is combined at the branching portion and output from the input section as an intensity modulated optical signal.
The second aspect provides an optical modulation apparatus comprising an optical signal input section, an optical path that propagates the optical signals, a branching portion that splits the optical path into a plurality of optical paths, an optical modulator that phase modulates optical signals in at least one of said plurality of optical paths, a wavelength selective filter that selectively reflects and transmits light: a structure whereby an appropriate time within a range having a lower limit obtained by subtracting one-fourth of an integer multiple of a high-frequency radio signal period applied to the optical modulator and an upper limit obtained by adding one-fourth of the integer multiple of said period is equal to twice the time it takes an optical signal to reach the filter from the optical modulating electrode feeding point; and a structure whereby an optical signal input from the input section that is divided into a plurality of optical paths at the branching portion, phase modulated by a phase modulator, reflected by the filter back along the optical path and again phase modulated by the phase modulator is combined at the branching portion and output from the input section as an intensity modulated optical signal.
The third aspect of the invention provides an optical modulation apparatus comprising an optical signal input section, an optical path that propagates the optical signals, a branching portion that splits the optical path into a plurality of optical paths, an optical modulator that phase modulates optical signals in at least one of said plurality of optical paths, a wavelength selective filter that selectively reflects and transmits light; an output section that outputs an optical signal transmitted by the filter; a structure whereby an appropriate time within a range having a lower limit obtained by subtracting one-fourth of an integer multiple of a high-frequency radio signal period applied to the optical modulator and an upper limit obtained by adding one-fourth of the integer multiple of said period is equal to twice the time it takes an optical signal to reach the filter from the center of the optical modulator; a structure whereby an optical signal input from the input section that is divided into a plurality of optical paths at the branching portion, phase modulated by a phase modulator and transmitted by the filter is output from the output section, and a structure whereby light that is reflected by the filter back along the optical path and again phase modulated by the phase modulator is combined at the branching portion and output from the input section as an intensity modulated optical signal.
The fourth aspect of the invention provides an optical modulation apparatus comprising an optical signal input section, an optical path that propagates the optical signals, a branching portion that splits the optical path into a plurality of optical paths, an optical modulator that phase modulates optical signals in at least one of said plurality of optical paths, a wavelength selective filter that selectively reflects and transmits light; an output section that outputs an optical signal transmitted by the filter; a structure whereby an appropriate time within a range having a lower limit obtained by subtracting one-fourth of an integer multiple of a high-frequency radio signal period applied to the optical modulator and an upper limit obtained by adding one-fourth of the integer multiple of said period is equal to twice the time it takes an optical signal to reach the filter from the optical modulating electrode feeding point; a structure whereby an optical signal input from the input section that is divided into a plurality of optical paths at the branching portion, phase modulated by a phase modulator and transmitted by the filter is output from the output section, and a structure whereby light that is reflected by the filter back along the optical path and again phase modulated by the phase modulator is combined at the branching portion and output from the input section as an intensity modulated optical signal.
In accordance with a fifth aspect, an optical signal of a first wavelength transmitted by the filter and an optical signal of a second wavelength reflected by the filter are input to the optical modulation apparatus.
In accordance with a sixth aspect, the optical modulator has a resonant type modulation portion.
In accordance with a seventh aspect, the modulator comprises a plurality of modulation portions disposed in series, at least one of which performs low-frequency signal modulation and another of which performs high-frequency radio signal modulation.
In accordance with an eighth aspect, the modulation apparatus includes transmission means for transmitting at least a portion of the optical signals output from the output section, receiving means for receiving a high-frequency radio signal, and modulation means that uses the optical modulator to modulate light with the high-frequency radio signal.
In accordance with a ninth aspect, the modulation apparatus further comprises an electrooptic converter that converts a portion of the light output from the output section to electrical energy that is supplied to transmission means, receiving means and modulation means.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.